Silencing button (4)
I believe the answer is:
mute
'silencing button' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mute that I've seen before include "Silent, refraining from speaking" , "having no voice" , "Silencer" , "Violin clamp; type of swan" , "Device for softening musical sound" .)